The success of rotary drilling enabled the discovery of deep oil and gas reservoirs. The rotary rock bit was an important invention that made rotary drilling economical. In drilling boreholes in earthen formations by the rotary method, rock bits fitted with one, two, or three rolling cutters are employed. The bit is secured to the lower end of a drill string that is rotated from the surface or by downhole motors or turbines. The cutters mounted on the bit roll and slide upon the bottom of the borehole as the bit is rotated, thereby engaging and disintegrating the formation material to be removed.
The roller cutters are provided with teeth or cutting elements that are forced to penetrate and gouge the bottom of the borehole by weight from the drill string. The cuttings from the bottom and sidewalls of the borehole are washed away by drilling fluid that is pumped down from the surface through the hollow drill string and are carried in suspension in the drilling fluid return to the surface.
One type of cutting element in widespread use is a cemented tungsten carbide insert which is interference pressed into an aperture in the cutter body or shell. Cemented tungsten carbide is a composite metal which is harder than the steel body of the cutter and has a cylindrical base portion and a cutting tip portion. The cutting tip portion is formed in various configurations, such as chisel, hemispherical or conical, depending upon the type of formation to be drilled.
Some of the cemented tungsten carbide inserts have very aggressive cutting structure designs and carbide grades that allow the bits to drill in both soft and medium formations with the same bit. These aggressive inserts are located in inner and heel rows which extend circumferentially around the cutter. The cutter also has a gage surface and a heel surface which is located at the outer edge of the inner rows and which joins the gage surface at an angle. Gage inserts are located on the gage surface to engage a sidewall of the borehole. In some cutters, scraper inserts are located at the junction between the heel surface and the gage surface for scraping the sidewall of the borehole.
During drilling high contact stresses and heat are generated, particularly by frictional engagement of the gage inserts and the scraper inserts with the borehole sidewall. Cemented tungsten carbide inserts contain a binder of a soft metal such as cobalt. Excessive heat can soften the binder, leading to plastic deformation of the insert under the high contact stresses common in drilling. The binder can also be chemically leached by the drilling fluids, or abraded away by the harder particles in the formation. All of the above conditions cause cracks to occur. These cracks can lead to premature failure. Composite carbides without binders are known, however, they are more brittle and thus subject to fracture more readily than a cemented carbide containing cobalt.